Sending email on profile update by admin. (1 post)

I am writing a custom plugin for my scout troop, and in the plugin I added ~10 roles for misc. positions in the troop. What I want to do is every time the admin promotes someone to a new role, the user who was promoted should get an email detailing that role's capabilities, so that the user knows what they can and can't do. For example, if I were to promote user JSmith to the role of Webmaster, JSmith should recieve an email saying:
As a Webmaster, you can:
cap_1
cap_2
etc

What hook should i use to detect the updating of the profile by an admin? I would use edit_user_profile, but I think that would send the email even if the user just changed their bio or something. I only want the email sent once, when the admin promotes the user.